High-side semiconductor switches are semiconductor switches that can switch a high supply voltage of several 100 V, such as 320 V direct current voltage. To this effect, they are connected between a positive supply voltage and a floating switching point, which, during switching operations, passes through a voltage range from ground potential to a potential close to the positive supply voltage. High-side semiconductor switches are used, for example, for driving electric motors at the mains voltage or in buck converters. They are generally realized as power transistors, such as MOSFETs (metal oxide field effect transistors) and IGBTs (bipolar transistors with insulated gate electrodes).
Control circuits for high-side semiconductor switches, referred to below in short as high-side control circuits, are described, for example, in “Motor Drive Control IC Designer's Manual” of the International Rectifier Corp., CA, USA (www.irf.com) and on the website www.innovatia.com/design_center/ under “high-side drivers.htm”.
A schematic diagram of a control circuit for a N-channel high-side MOSFET is shown in FIG. 1. FIG. 1 shows an example of a directly-coupled high-side control circuit for a MOSFET or IGBT.
The high-side control circuit shown in FIG. 1 comprises a high-side semiconductor switch 10 that is to be switched, taking the form of a N-channel MOSFET in the illustrated embodiment. The high-side semiconductor switch 10 is connected between a supply voltage 12 and a floating switching point 14. The floating switching point 14 may be connected to ground via a load or a low-side MOSFET (not illustrated in the figure). Four function blocks are provided for driving the high-side semiconductor switch 10:
A circuit 16 for providing a drive voltage for the high-side semiconductor switch 10; a driver circuit 18 for driving the high-side semiconductor switch on the basis of the drive voltage, particularly for charging and discharging the gate of the high-side MOSFET 10; an input circuit 22, that is related to ground and establishes the connection to a control logic, such as a microcontroller; and a level shift circuit 20 that transforms the ground-related control signal into a floating voltage level for the driver circuit.
In order to switch on a N-channel MOSFET and to keep it in a switched-on state, it is necessary to have a voltage potential that is higher than the potential of the positive supply voltage 12. The gate voltage of the high-side MOSFET has to be approximately 10 to 15 V higher than its drain voltage so as to enable the high-side MOSFET to be switched on reliably. However, since the positive supply voltage is applied to the semiconductor switch, the gate voltage has then to be higher than the positive supply voltage, which is frequently the highest voltage that is available in the system. In the prior art, a bootstrap circuit is thus often employed. A bootstrap circuit basically consists of a capacitor 24 that is connected by one of its terminals to the floating switching point 14 and by its other terminal it is connected via a diode 26 to an auxiliary voltage 28. The auxiliary voltage should be between +10 or +15 V. While the high-side semiconductor switch 10 blocks conduction, the bootstrap capacitor 24 is recharged from the auxiliary voltage 28 via the diode 26. The voltage generated at the bootstrap capacitor 24 is not related to ground but rather to the voltage of the floating switching point 14 and is thus also referred to as a floating voltage.
It is further necessary for the gate voltage to be controllable via a logic circuit that is normally related to ground. The control signal level has therefore to be shifted to the level of the source of the high-side semiconductor switch (level shift), this source voltage corresponding to the floating switching point and in most applications oscillating between the positive and the negative supply voltage or respectively between the positive supply voltage and ground.
The driver circuit 18 is supposed to effect the quickest possible charge and discharge of the gate capacitor of the high-side semiconductor switch 10 and is frequently realized by a push-pull circuit having two alternately switching transistors 30. Like the bootstrap capacitor 24, the driver circuit 18 is also based on the floating switching point 14.
The input circuit 22 is related to ground potential 32 and receives a control signal from a control logic, such as from a microcontroller that is not illustrated in the figure. Should the high-side control circuit be used, for example, for driving an electronically commutated DC motor, the control logic may generate the necessary commutation signals. Should the high-side control circuit be used in a buck converter, the control logic predetermines, for example, a desired duty cycle, a desired ON time and a desired cycle duration, in order to adjust the output voltage through the regulated switching on and off of the high-side semiconductor switch.
The level shift circuit 20 serves to transform the ground-related control signal into a floating voltage level for the driver circuit 18. To achieve this, in the prior art, a current is always conducted to ground through a semiconductor switch or a current drain. The current causes a voltage drop in a resistor that is drawn on for driving the driver circuit 18.
The power received by the high-side control circuit should not perceptibly worsen overall efficiency.
Basically, there is the option of having an inverting or a non-inverting control for a high-side semiconductor switch. In the case of the inverting control, the high-side semiconductor switch is switched on when a lower logical voltage level “Low” is applied at the input circuit 22 (Low active). Correspondingly, for a non-inverting control, a higher logical voltage level “High” is required in order to switch on the high-side semiconductor switch 10 (High active). Integrated circuits (IC) are available from various semiconductor manufacturers, such as International Rectifier Corp., in which the above-described function blocks illustrated in FIG. 1 are integrated. An example of this kind of IC is the IR 2110 from International Rectifier Corp. However, the individual function blocks in these ICs are distinctly more complex than as described above and the ICs generally have additional functionalities. In order to provide the drive voltage for the high-side semiconductor switch 10, the user has furthermore to add on, in a discrete circuit, at least the bootstrap capacitor 24 and the diode 26. The known integrated circuits (ICs) are generally well suited for use, for example, in motor electronics. The disadvantage is the high price compared to a purely discrete circuit.
For discretely constructed high-side control circuits, there are often difficulties for motor applications in the region of the level shift circuit that are not acceptable. In the case of a high-active control, it is thus necessary to conduct a current to ground during the ON time of the high-side semiconductor switch, which, for high supply voltages, results in considerable losses and associated component heating. For low-active control circuits, a similar problem arises during switch-off of the high-side semiconductor switch when it directs a current in the reverse direction (reverse current) through its inverse diode, as can happen in motor applications when, after commutation, an electric motor in generator mode feeds back current into the control circuit for a short time. The integrated circuits available on the market solve this problem in that a flip-flop is connected upstream of the driver circuit, which, through a short pulse, can be set or reset respectively. In this solution, a current has only to be conducted to ground during the length of the pulse, whereby losses can be kept low.
Based on this prior art, it is an object of the invention to provide a control circuit for a high-side semiconductor switch for switching a supply voltage, the control circuit being discretely constructed using as few components as possible and thus being cost-effective and nonetheless generating the lowest possible losses during switching operations and even when a reverse current flows through the high-side semiconductor switch. The control circuit is to be preferably used in motor applications and needs a sufficiently rapid switching speed and reliable maintenance of the voltage level in stationary switching states.